1. Field of the Invention
The present invention relates to the field of x-ray imaging technology, and more particularly, to electronic readout of x-ray images.
2. Background Information
A variety of techniques are presently available for obtaining x-ray images. One common technique employs an x-ray absorbing phosphor screen which emits optical radiation which exposes photographic film held adjacent the phosphor screen. This technique offers the advantage of high resolution, but is not effective in real time because of the need to develop the photographic film to obtain a visible x-ray image.
Another alternative is an x-ray image intensifier tube in which x-rays are absorbed by a fluorescent screen which emits photons which are in turn absorbed in a layer of photoelectron emitting material which emits electrons which are then accelerated and focused on a phosphor screen to produce a higher intensity visible image. While this system operates in real time, it suffers from the disadvantage that it produces relatively low resolution images as a result of optical scattering, imperfect electron optics, loss of sharpness in the optics coupling the image intensifier to the camera and other effects. In addition, it is bulky, fragile, expensive and requires high voltage to operate.
U.S. Pat. No. 4,011,454 to Lubowski et al., entitled "Structure X-Ray Phosphor Screen" which is incorporated herein by reference in its entirety, discloses a modified x-ray image intensifier which provides increased resolution through the use of a structured scintillator material as the fluorescent screen. This structured scintillator screen is produced by a vacuum evaporation process in which CsI is evaporated from a source boat and deposited on a topographically structured surface to produce columnar scintillator elements. During the deposition, the structured surface is maintained at a temperature in the range of 50.degree. C. to 150.degree. C. The scintillator is then fired at 450.degree. C. to 500.degree. C. to compact the scintillator. The deposition process is then repeated to produce taller scintillator elements. This is followed by another firing at 450.degree. C. to 500.degree. C. to compact the scintillator. Following the final deposition, the scintillator is fired at 615.degree. C.
In recent years, the art of electronic image processing has advanced rapidly. These advances have made computed tomography (CT) machines not only feasible, but very valuable medical diagnostic tools. However, such machines are substantially larger and more expensive than typical x-ray machines and are more suitable for obtaining images of slices through the body rather than a chest x-ray type of image of the body.
There is a need for high resolution x-ray imaging systems which have an improved modulation transfer function (MTF). The modulation transfer function is the output contrast divided by the input modulation and is a function of the spatial frequency of the modulation.
Semiconductor photosensitive imaging arrays are widely available today. They are used in television cameras, facsimile machines and a wide variety of other applications. These photosensitive imaging arrays can be made with a resolution of more than five line pairs per millimeter and thus are capable of providing high resolution conversion of visible light images to electronic form. Unfortunately for the x-ray art, such photosensitive imaging arrays do not respond to x-ray radiation and are much too small for effective use in x-ray imaging applications.
There is a need for increased resolution in real time x-ray imagery for x-ray images of the type typically provided on x-ray film and for electronic output of the x-ray image rather than optical output to facilitate electronic processing of the image data.
Related, incorporated by reference Application Ser. No. 07/590,848 provides such an array by growing a structured scintillator array on a photosensitive imaging array. While such a technique is effective, it can suffer from two disadvantages. First, the need to grow the scintillator structure on the photosensitive imaging array restricts the processing steps which can be used to fabricate the imaging array and to grow the scintillator array. Second, any errors which destroy the imaging array during fabrication or result in an unusable scintillator array increase the manufacturing costs because of the resulting loss of a working photosensitive imaging array or scintillator structure.
Accordingly, there is a need for increased flexibility in the provision of such x-ray imaging arrays.